A power circuit, in particular DC (Direct Current)-power circuits that work with high voltages and high currents, for example in photovoltaic systems or systems providing off-grid power supply, are prone to the development of electric arcs. Electric arcs can for example occur when a power line with a high current load is interrupted during maintenance or in case contacts at interconnectors are degraded. Other possible courses for arc faults, i.e. the occurrence of an electric arc in the power circuit, are corroded solder joints or broken insulators of the power lines. Arc faults are the most common fire causes in photovoltaic systems. This also reflects in the requirements for arc fault protection for photovoltaic systems as for example regulated by the National Electric Code (NEC) 690.11 of the United States of America coming into force in 2011.
A reliable arc fault detection method and system is therefore of major importance. On the one hand, for security reasons the existence of an arc fault has to be detected with a reliability as high as possible. On the other hand, the probability of an erroneous indication of a supposed arc fault has to be as low as possible, in particular if an erroneous detection of an arc fault might lead to a shutdown of a photovoltaic system without the option to automatically restart it, as for example specified in the before mentioned NEC 690.11 code.
Electric arcs usually emit a broadband AC (Alternating Current)-signal in an RF (Radio Frequency)-frequency range. Detection systems for arc faults that are based on detecting an according radio frequency signal in the power circuit are well established and for example known from document WO 95/25374. A problem associated with the detection of arc faults via their AC-current signature can arise from the weakness of the signal. In order to ensure that arc faults are assuredly detected, detection circuits with a high detection sensitivity are required. A down-side of high detection sensitivity could be an increased number of nuisance alarm situations, in which a noise signal, e.g. due to a disturbing RF-signal that is coupled into the detection circuit, is misinterpreted and wrongly assigned to the presence of an arc fault. Possible sources of disturbing signals are for example RF-radio transmitter, electric trains or trams passing by, electric or electronic devices with an insufficient electro-magnetic shielding or interference suppression, or arcs in adjacent power systems. In the following, all possible RF-sources other than an arc fault in the respective power system are hereinafter referred to as a “disturber”.
It is thus desirable to create a method and system for reliably detecting arc faults in a power circuit. It is furthermore desirable to describe a photovoltaic system with a corresponding detection system.